Hollow structures have received great attention due to their wide applications in many fields such as catalysis, encapsulation, delivery and controlled release of drugs, chemical/biological separation and sensing. The most-applied method for the synthesis of hollow structure is by far the templating approach, including two main pathways. One is the templating against colloidal particles; the basis of this process is coating the template particles in solution by direct surface reactions that utilize specific functional groups on the cores to create core/shell composites, followed by removal of the cores by calcination or solvent extraction. The other is the layer-by-layer self-assembly technique, in which polyelectrolyte and micrometer sized inorganic nanoparticles are usually used as sacrificial cores.
Although tremendous success has been made in synthesizing hollow structures by the templating approach, the limitations of such method are also obvious: “hard templates” which require complicated preparation sequences are always needed, thus significantly increasing the cost for producing hollow structures at large scale.